Do you mean the plug in 110V block heater?
If so you only need it on for about an hour to get the engine plenty warm for a easy start.
Years ago Ny tow truck was plugged in on a timer with a hour on,hour off,hour on,.... it started easy at any time. I was just saving money on the 600 watt usage.
(RVM#26) THE U-RV 94 F-700/24 foot U-haul box home built RV